Everyone learns using their own methods but this is how I started (probably not the best):
If you just want to develop new packages, having the ER model of core and packages is helpful, but how much of it one will need to study it depends on what your package does.
As for (3), studying the templating system docs plus some examples from other packages is enough to get you started.
this link does not work (perhaps due to a missing slash at the end). Once found, this page contained another broken link, http://www.brasileiro.net/writings/openacs/oacs-4-6-small.jpg
but I've found the page at:
I am using MSIE 6.0 and my ISP does IP masquerading.